It could also be a group of vengeful spirits in Tartarus. They were once defeated by Percy or his friends, and now they see the opportunity to get revenge on him while he's in their territory. They could band together and use their combined powers to try and make Percy's life in Tartarus a living hell.
Maybe an old and powerful monster that has long been trapped in Tartarus could be the main antagonist. It sees Percy as a threat or a source of entertainment, and it tries to stop him from finding a way out or from getting stronger.
Nyx could be a great main antagonist. She's a primordial goddess of the night and Tartarus is her domain in a way. She might see Percy and his friends as intruders and try to stop them from leaving or achieving their goals. She could use her power over darkness and the unknown to create all sorts of obstacles for them.
Perhaps one of the Titans who were banished to Tartarus. They might see Percy as a threat to their plans of revenge or escape. Kronos, if he still has some form of existence in Tartarus, could be a major antagonist. He would have his own motives and powers, and he could try to manipulate Percy or simply get rid of him.
It could be Nyx. She is a very powerful primordial goddess in Tartarus. Since Percy is alone, she might see him as an easy target or a threat to her domain. Nyx could send all sorts of dark creatures and illusions to stop Percy from achieving whatever goal he has in Tartarus.
In a 'percy banished to tartarus fanfic', the plot could be about Percy's internal battle as well. He's been banished, which could make him feel betrayed and angry. So he not only has to deal with the physical threats in Tartarus but also his own emotions. He could question his identity and his purpose, and this self - discovery could be a major part of the story. He might start to see things from a different perspective, and this new understanding could be crucial in his journey to either escape or change his situation in Tartarus.
The main antagonist could be Nyx. She is a powerful primordial goddess of the night, and Tartarus is her domain in a way. She might see Percy as an intruder and try to stop him from escaping or achieving his goals there.
The monsters native to Tartarus could also be antagonists. These could be huge, grotesque creatures that have never seen the outside world. They might attack Percy on sight, seeing him as prey. Some of them could be powerful enough to even give Percy a hard time, especially if his powers are weakened in Tartarus.
It could be another powerful god or goddess who wants to use Percy's betrayal of Tartarus for their own gain. Maybe a god of chaos who sees this as an opportunity to create more disorder in the world. They might manipulate Percy into the betrayal and then turn on him later.
The main antagonist might be a new character, a powerful being that rules the realm where Percy is banished. This being could see Percy as a threat to his or her power and try to stop Percy from finding a way back.
A powerful sorcerer who also wants to claim the island for his own could be the main antagonist. He sees Percy as an obstacle to his plans and tries to eliminate him. The sorcerer might have control over some of the dangerous creatures on the island and use them to attack Percy.
The main antagonist might be an ancient and powerful being that has awakened. This being could see Percy and Annabeth as a potential threat to its plans of taking over the demigod world. So, it uses its influence to have them banished, thinking that without them, it would be easier to conquer. For example, a long - forgotten Titan who has found a way to break free from its prison.